Past implementations of hybrids used to accomplish four-wire to two-wire conversion employ a transmission signal cancellation path to duplicate and cancel the portion of the outgoing transmission signal that is erroneously picked up by the receive amplifier. A hybrid used in a typical solid-state Data Access Arrangement (DAA) device will employ a gain stage that serves as the transmission signal cancellation path.
Telephone lines present a wide range of characteristic impedances. The characteristic impedance varies with frequency on a particular telephone line, and each two-wire subscriber line, or telephone line, will present a different characteristic.
Hybrids in present DAA devices use a fixed gain stage for cancellation that is not responsive to changes in the telephone line impedance. Such a fixed gain stage does not provide frequency dependent impedance matching. The DAA is designed to perform a coarse echo cancellation by providing a hybrid that is not precisely balanced. The prior art DAA devices do not provide for the connection of external impedance matching devices.
In view of recent telecommunication techniques that utilize a larger portion of the available bandwidth such as that specified by ITU-T recommendation V.90, it would be advantageous to provide a transmission signal cancellation path in a DAA device that is more flexible and responsive to varying telephone line conditions, and which is able to better model the telephone line impedance to provide better echo cancellation, especially at the band edges.